Modelling thermal sensitivity in the full phenological distribution: a new approach applied to the spring arboreal caterpillar peak

Advances in spring phenology are among the clearest biological responses to climate warming. There has been much interest in how climate impacts on phenology because the timings of key events have implications for species interactions, nutrient cycling and ecosystem services. To date most work has focused on only one aspect of population phenology, the effects of temperature on the average timing. In comparison, effects of temperature on the abundance of individuals and their seasonal spread are understudied, despite their potential to have profound impacts on species interactions. Here we develop a new method that directly estimates the effect of spring temperatures on the timing, height and width of the phenological distribution and apply it to temperate forest caterpillars, a guild that has been the focus of much research on phenology and trophic mismatch. We find that warmer spring conditions advance the timing of the phenological distribution of abundance by −4.96 days °C ⁻¹ and increase its height by 34% °C ⁻¹ but have no significant effect on the duration of the distribution. An increase in the maximum density of arboreal caterpillars with rising temperatures has implications for understanding climate impacts on forest food chains, both in terms of herbivory pressure and the resources available to secondary consumers. The new method we have developed allows the thermal sensitivity in the full phenological distribution to be modelled directly from raw data, providing a flexible approach that has broad applicability within global change research.